There are some MP database functions run SQLite functions but I'm afraid some of them don't. I did not code MyPlaza, ZiNgA BuRgA did. I have ported it and updated it to work with MyBB 1.4.x.
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After taking a look at myplaza_install.php and myplaza_general.php looks like most of the functions should support SQLite because they use MyBB's $db->write_query.
I'm not sure if the queries run through that function work with SQLite but you can try it and report back

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